So-called laser produced plasma (LLP), EUV light sources, or EUV radiation producing apparatuses include a driver laser system that includes a beam source, for example, a CO2 laser, for producing pulsed laser radiation (i.e., a sequence of laser pulses). The pulsed laser radiation is typically amplified by a plurality of optical amplifiers of an amplifier arrangement of the driver laser system. The laser radiation produced and amplified by the driver laser system is fed via a beam guiding device to a focusing device, which focuses the laser radiation in a target region at which is provided a target material, for example in the form of tin droplets, which upon irradiation with the laser radiation transitions to a plasma state and emits EUV radiation in the process.
In the case of such an EUV radiation producing apparatus, a portion of the pulsed laser radiation is reflected back from the target material and passes through the beam guiding device and the optical amplifiers of the amplifier arrangement in the reverse direction, such that the back-reflected laser radiation is likewise amplified. As a result, the gain of the amplifier medium in the optical amplifiers decreases, such that the maximum achievable power during the amplification of the laser radiation propagating in the forward direction, i.e., in the direction of the target material, is reduced. Typically, it is also necessary to filter the back-reflected laser radiation to protect the beam source from destruction by the back-reflected laser radiation. If conventional optical isolators, e.g., in the form of Faraday isolators or in the form of electro-optical modulators are used, this is possible only up to a limited maximum power of the laser radiation, because the optical isolators are otherwise damaged by the back-reflected laser radiation.
WO 2015/082004 A1 discloses an amplifier arrangement for example for a driver laser system of an EUV radiation producing apparatus, wherein the amplifier arrangement includes an optical isolator including a polarizer device and a phase shifting device. At least the polarizer device is positioned at a location at which the laser radiation has a laser power of more than 500 W.
As a result of the optical filtering in an optical isolator configured as described above, a thermal lens is typically produced in the filtering element of the optical isolator, which thermal lens disadvantageously alters the divergence or the beam properties of the forward-traveling laser radiation or of the forward-traveling laser beam. If the maximum power limit of the filtering optical element is attained, moreover, the EUV radiation producing apparatus or the EUV lithography apparatus has to be shut down. Since a restart of the EUV lithography apparatus causes a significant loss of time, the productivity of the EUV lithography apparatus is reduced as a result.
An added difficulty in such systems is that optical isolators based on the polarization or phase shift principle, in a manner governed by the design, can suppress laser radiation only upon the occurrence of a specific sudden phase change or a specific phase shift (e.g., 180°). The value for such sudden phase change or phase shift is possibly not complied with upon reflection at a droplet, such that for this reason, too, the laser radiation reflected at the droplet cannot be completely suppressed by such an optical isolator.
WO 2015/045102 A1 describes a laser apparatus including a master oscillator, which emits a pulsed laser beam. The pulsed laser beam is amplified in a plurality of optical amplifiers arranged in the optical beam path of the pulsed laser beam. The laser apparatus can additionally include a light reflector that allows the pulsed laser beam to pass through it, wherein the light reflector reflects self-oscillating radiation produced at one of the plurality of amplifiers. The laser apparatus also includes a radiation absorber, which takes up and absorbs the self-oscillating radiation reflected by the light reflector.
US 2015/0208494 A1 describes an EUV light source in which a target material is provided at a target position and an amplified laser beam that propagates in the direction of the target position is focused in a focal plane. The target position is situated outside the focal plane and an interaction between the amplified laser beam and the target material converts at least part of the target material into a plasma that emits EUV radiation. Back-reflections in the EUV light source are intended to be reduced in this way.
U.S. Pat. No. 4,194,813 discloses an optical isolator for suppressing back-reflections emanating from a target irradiated by means of a laser. The optical isolator includes a disk, e.g., composed of aluminum or tantalum, arranged in a vacuum chamber, with a diaphragm aperture arranged in the beam path of the laser. The beam path is guided through the chamber. The isolator additionally includes means for focusing laser radiation onto the edge of the diaphragm aperture to damage the edge of the diaphragm aperture and to generate a plasma in the process. U.S. Pat. No. 4,194,813 also specifies that other methods are known for protecting a CO2 laser beam from the back-reflection thereof at a target, wherein the methods usually include the formation of a plasma discharge in air. One such method includes a discharge at a foil composed of Mylar® by means of the laser pulse itself or by means of an auxiliary laser beam. Other methods include a gas discharge in the vicinity of a focal point, wherein a reflector is used to concentrate the energy of the marginal region of the laser beam at a center, such that the electric field of the laser beam exceeds a plasma ignition threshold in air. Elsewhere, discharges are intended to be produced by applying an overvoltage pulse to a small region, wherein the pulse is synchronized with the laser beam.
In addition to the problem of back-reflections during the amplification of laser pulses in an optical amplifier arrangement or generally in an optical amplifier, so-called self-oscillations (“self-lasing”) or so-called “amplified spontaneous emission” (ASE) can occur, i.e., amplified spontaneous emissions that can lead to the limitation of the gain in a respective amplifier stage and, in the worst case, even to the destruction thereof. ASE increases the signal background, such that even without the presence of an input signal to be amplified in the form of a pulsed laser beam or in the form of laser pulses in an optical amplifier, radiation is generated. Although self-lasing in an amplifier arrangement can be limited by reducing the reflective surfaces in this way to reduce the number of passes, in general it cannot be completely suppressed.